Chickpea Roots Undergoing Colonisation by Phytophthora medicaginis do not Display Hormone Accumulation nor Signalling Typically Associated with Model Leaf Hemibiotrophic Diseases

As a broad group, hemibiotrophic pathogens cause significant losses within agriculture threatening the sustainability of food systems globally. The complex manner in which these microbes colonize their hosts, including an initial biotrophic phase of colonization followed by a biotrophic-to-necrotrophic switch (BNS) phase and ending with a necrotrophic mode of nutritional acquisition, renders their management more complex than pathogens which display a single nutritional strategy. Within model plant systems, typically each of these phases is characterized by both common and discrete transcriptional responses as the host modulates physiological processes in concert with changes in pathogen biology. Plant hormones also play an important role in these stages with classic models showing that salicylic acid accumulates during the biotrophic phase and jasmonic acid/ethylene responses occur during the necrotrophic phase. In this study, we use the interaction between the hemibiotroph Phytophthora medicaginis and the roots of its host Cicer arietinum (chickpea) to define the duration of the different life stages of P. medicaginis during the pathogenesis of chickpea.  Using transcriptional profiling across all three stages of infection we demonstrate that chickpea displayed some similarities in response to P. medicaginis as has been previously documented in other model plant-pathogen hemibiotrophic interactions.  However, our transcriptomic results suggest that chickpea does not conform to the phytohormone response model observed in leaf colonization, nor to that observed for roots of other plant species being colonised by soil-borne hemibiotrophs. We confirm these findings using targeted quantification of salicylic acid and jasmonic acid. The findings from this study demonstrate that a wider spectrum of plant species should be investigated in future to understand the physiological changes in plants during the different stages of colonization by hemibiotrophic soil-borne pathogens before we can better manage these economically important microbes in agronomically relevant conditions. In total 16 samples were analysed by RNA-sequencing of which 12 samples included 4 replicates each of 3 timepoints 12, 24 and 72 hours post P. medicaginis inoculation in chickpea var. 'Sonali' roots encompassing three phases of infection namely biotrophic, biotrophic to necrotrophic switch and necrotrophic phases, respectively. The remaining four samples included RNA from mock-inoculated chickpea roots inoculated with agar blocks without oomycete mycellium overlaid on cellophane membranes as well as axenically grown P.medicaginis grown on V8 juice media supplemented with CaCO3 before transferring to full stength MS media supplemented with CaCO3 for 12 hours before harvesting.